Sizing is a term used in the papermaking art to describe processes which reduce the water absorbency of a paper sheet. Functionally, a sized paper sheet resists wicking by water-based ink applied to the sheet surface. Sizing also improves the dimensional stability of a sheet by inhibiting absorption of atmospheric moisture.
Sizing effectiveness in paper is measured by either or both of two standardized edge-wicking tests wherein the face surfaces of a paper sample are protected by waterproof tape and the exposed edge sample immersed in a penetrating solution for a measured time interval. Afterward, the sample is weighed and the value obtained is compared with the preimmersion sample weight to determine the quantity of solution absorbed by the sample. This absorbed quantity is then normalized by the edge area of the sample
One such edge-wicking test utilizes a 35% solution of hydrogen peroxide as the penetrating solution. The other such test subjects the sample to a 1% solution of lactic acid. Depending on the utility of the paper product, one test may be more significant than the other. For example, paper used for milk containers must have a low capacity for lactic acid edge-wicking.
Historically, sizing agents have been formulated from a mixture of about 1% per ton of dry pulp natural, anionic rosin, and about 1.5 to 2% alum (Al.sub.2 SO.sub.4).sub.3. In an acidic papermachine headbox furnish of about 4.0 to 4.5 pH, these compounds coprecipitate onto the cellulose fiber to be subsequently stabilized by drying to form a hydrophobic coating. This process of blending the size formulation with the headbox furnish is characterized as "internal sizing" due to the fact that the sizing is distributed homogeneously throughout the thickness of a paper web formed from such headbox furnish.
Although natural anionic rosin sized paper formed from an acidic headbox furnish has good hydrogen peroxide holdout, the lactic acid holdout is normally poor.
Supplemental to the internal size, paper manufactured for converted utility as a liquid or beverage container is frequently "surface sized" with a solution of glue and/or starch. In such cases, the size solution is coated onto the surface of a dry web as the web runs into a pond of the solution confined between the web surface and a roll or doctor blade surface. When applied to both web surfaces simultaneously, respective ponds are confined between opposite web surfaces and respective members of a roll nip pair. This common arrangement is characterized as a "size press."
More recently, synthetic sizing agents such as alkyl ketene dimer, stearic anhydride, and alkenyl succinic have been developed to form true chemical covalent bonds with cellulose rather than the ionic or polar bonds of natural size. Most prevalent of these synthetic size compounds is alkyl ketene dimer (AKD).
Once cured, synthetic size is more stable against water, acids, and alkalis. Consequently, synthetically sized paper has good lactic acid holdout but normally poor hydrogen peroxide holdout. The process solution of synthetic size is acid/alkali sensitive, however, and, when used as an internal size, must be blended to a substantially neutral 6.5 to 8.5 pH headbox furnish. This circumstance gives rise to the trade characterization of "neutral sizing." Synthetic size has also been used as a surface size constituent; following a synthetic or "neutral" internal size treatment, however.
Although synthetic size may be blended with cationic resins in an internal sizing process to improve hydrogen peroxide holdout, the necessary neutral pH headbox solution limits available brightness. Distinctly acid pulps are required for paper of the greatest brightness value.
It is, therefore, an object of the present invention to provide a paper sizing process by which high brightness values, low bacteriological contamination, and good holdout against hydrogen peroxide and lactic acid may be obtained.